Day 51: The Inverse Square Law

Welcome to day 51 in 100 days of photography! We are officially in the second half and I am going to kick it off with ten days of lighting. I want to start by telling you about my first ever experience with a flash. I was going to be testing the event waters at a vow renewal. It was only three hours of shooting but half of that time would be after the sun went down, so I rented a flash and a 70-200. I was feeling amazing, walking around with a huge lens attached to my Canon Rebel and a flash mounted right on top. As I was walking through the church lobby, my flash fell off my camera. I was such a newbie with flash that I didn’t even mount it right. In a panic, I caught the flash mid fall and looked around, embarrassed and making sure no one saw.

Flash is a whole new world. When we get our cameras, we learn how to find and use beautiful natural light. We struggle until we find success shooting in manual, nailing our exposure every time. Then, the sun goes down and we’re forced to face the facts that flash is a necessity when it comes to wedding photography.

In an effort to make flash less scary to those just starting out, I want to share what someone once told me. I was depressed at the fact that sunset didn’t last six hours like I needed it to and my friend told me the following:

“You’re so comfortable shooting with available light. What if I told you that you could move the sun? What if I told you that you could add an extra sun? What if you could control the power of the sun? What if you could create your own clouds… the size and shape of them even?”

All of a sudden I was on cloud nine. You see, your flash is your sun, and your modifiers are your clouds. Flash gives you a huge amount of control when you’re not afraid to use it.

Before day two of this lighting series, I want to explain the inverse square law. This was a huge game changer for me and although I had heard it ten times before, it didn’t click until a lighting workshop I took. The funny thing is, it wasn’t even the teacher who was able to explain it to me, it was another student. When you’re shooting with flash and it’s not working, it’s important to be able to identify why it’s not working and how you can fix it. This is where the inverse square law is worth knowing.

The inverse square law says this:

The inverse square law describes the intensity of light at different distances from a light source. Every light source is different, but the intensity changes in the same way. The intensity of light is inversely proportional to the square of the distance.

Okay, that makes very little sense. Basically the inverse square law tells us that the closer the light source is to a subject, the more intense the light and the quicker the light falls off. As you move the light source away from the subject, the spread of light is more even across the subject. You can see this in action. Try holding a flashlight up to your face in a dark room. The closer you hold the flashlight to one side of your face, the faster the light falls off. You’ll see a harsh light on your cheek and then the light barely reaches your nose. BUT when you move the flashlight away from your face, the beam of light spreads more evenly over your face. You also do this with your cell phone when you’re looking for something in the dark. If you don’t see what you’re looking for, you hold the phone high over your head to spread the beam of light over a greater distance. The light isn’t as intense because its farther away from the subject, but it spreads the light out over more space.

Check out the image below from, you can see a bright intensity of light very quickly and then from 1/36-1/100 there isn’t much change in lighting intensity at all.

You can also see this in these two images of Jessica. The light was so incredibly close to her face on the left that it was just outside of my frame. It gave me just enough light for the outline of her face and then, because of the inverse square law, everything else was very dark. Then I moved my light away from her face and turned up the power and the light went farther and spread evenly over her face. This is the effect your lighting distance has on your images. If you’re trying to light an entire dance floor and the light you’re using is right up against your floor, there’s a good chance the people on the one side will be very brightly lit while the people on the far side will be dimly lit. To fix that, you simply move your light far away so that the entire dance floor is standing within that 1/36 – 1/100 range.

Still not there? Not to worry, we are going to take care of that right now.

Have you ever been standing near train tracks? You hear a train and you’re actually not sure which way it’s coming from. Actually… is it even coming or going? It’s hard to tell, the train is so far away and the sound is so even. As the train gets closer, the sound gets louder and louder and faster and faster until in a frightening moment it whips past your ear and for just a split second, it’s shockingly loud. The sound falls off super fast because it’s so close to you, so that punch of sound lasts only a second. As the train moves away, the sound starts becoming more even until it’s just a faint, steady hum. You’ve experienced the inverse square law with sound. Light does the same thing.

This helps when you’re trying to light an entire dance floor and you know the light source should be farther away if you’re trying to light everyone evenly. This also helps when you want to create a dramatic portrait and want a sharp gradient falling across a subjects face. Knowing the inverse square law is one of the first steps to being able to create your own light with flash.

Friends, that was day 51! I will see you tomorrow for day 52: Flash gear and modifiers.